📚 Carboxylic Acids Exam-Focused Revision | 羧酸 考点精讲
Carboxylic acids are a fundamental homologous series in A-Level Chemistry, characterised by the –COOH functional group. This article presents a structured, exam-focused breakdown of their nomenclature, structure, physical properties, acidity, and key reactions, complete with comparisons to alcohols and phenols, inductive effects, and mechanistic details required by the CIE syllabus.
羧酸是A-Level化学中一个基本的同系物系列，其特征是–COOH官能团。本文紧扣CIE考纲，系统梳理羧酸的命名、结构、物理性质、酸性及关键反应，兼具与醇和酚的比较、诱导效应以及考试必需的机理细节。
1. Introduction and Nomenclature | 介绍与命名
Carboxylic acids are organic compounds containing the carboxyl group –COOH. Their IUPAC names are derived by identifying the longest continuous carbon chain that includes the carboxyl carbon, and replacing the terminal '-e' of the corresponding alkane with '-oic acid'. The carboxyl carbon is always assigned position 1. For example, HCOOH is methanoic acid, CH₃COOH is ethanoic acid, and CH₃CH₂COOH is propanoic acid. When substituents are present, the chain is numbered starting from the carboxyl carbon, and prefixes such as hydroxy-, chloro- or amino- are used with appropriate locants. Aromatic carboxylic acids are named as derivatives of benzoic acid, e.g. 2-hydroxybenzoic acid. Dicarboxylic acids use the suffix '-dioic acid', for instance HOOC–COOH is ethanedioic acid.
羧酸是含有羧基–COOH的有机化合物。其IUPAC名称通过找到包含羧基碳的最长连续碳链，并将相应烷烃词尾的“-e”替换为“-oic acid”（中文为“酸”）来命名。羧基碳总是编号为1。例如，HCOOH是甲酸，CH₃COOH是乙酸，CH₃CH₂COOH是丙酸。如有取代基，从羧基碳开始给碳链编号，并加上如羟基-、氯-或氨基-等前缀和相应的位次。芳香族羧酸作为苯甲酸的衍生物命名，例如2-羟基苯甲酸。二元羧酸使用后缀“-dioic acid”（中文为“二酸”），例如HOOC–COOH是乙二酸。
2. Structure and Bonding | 结构与化学键
The carboxyl group consists of a carbonyl (C=O) and a hydroxyl (–OH) attached to the same sp² hybridised carbon. The carbon–oxygen double bond is shorter than a single bond, and the C–O– close to 120° around the carbonyl region. The lone pair on the hydroxyl oxygen undergoes p– n deprotonation, and has profound consequences for the acidity and reactivity of the acid.
羧基由一个羰基（C=O）和一个羟基（–OH）连接到同一个sp²杂化碳上组成。碳氧双键比单键短，C–O–H键角在羰基区域接近120°。羟基氧上的孤对电子与羰基π体系发生p-π共轭，使电子密度离域。这种离域作用使去质子化后生成的羧酸根负离子稳定化，并对羧酸的酸性和反应性产生深远影响。
3. Physical Properties | 物理性质
Carboxylic acids exhibit significantly higher boiling points than alcohols of comparable molar mass. This arises from their ability to form strong intermolecular hydrogen bonds: two carboxylic acid molecules can dimerise through a pair of O–H···O hydrogen bonds, creating an eight-membered ring dimer. As a result, methanoic acid (boiling point 101 °C) and ethanoic acid (boiling point 118 °C) have much higher boiling points than ethanol (78 °C) despite similar molecular sizes. Lower-molecular-weight carboxylic acids (up to butanoic acid) are completely miscible with water, again due to extensive hydrogen bonding with water molecules. Solubility decreases as the hydrocarbon chain lengthens, because the hydrophobic alkyl part dominates.
羧酸的沸点显著高于相对分子质量相近的醇。这是因为它们能形成强分子间氢键：两个羧酸分子可以通过一对O–H···O氢键二聚，形成一个八元环二聚体。因此，甲酸（沸点101 °C）和乙酸（沸点118 °C）的沸点远高于乙醇（78 °C），尽管分子大小相近。低分子量的羧酸（最多到丁酸）与水完全互溶，同样是由于与水分子之间形成大量氢键。随着烃链增长，溶解度下降，因为疏水的烷基部分占主导地位。
4. Acidity of Carboxylic Acids | 羧酸的酸性
Carboxylic acids are weak acids with pKa values typically in the range 4– e relatively high acidity is attributed to the resonance stabilisation of the carboxylate ion: the negative charge is delocalised equally over both oxygen atoms, giving two equivalent resonance structures. This delocalisation lowers the energy of the conjugate base, making the acid more willing to release a proton. In contrast, alkoxide ions lack resonance stabilisation, and phenoxide ions have resonance but the negative charge is delocalised into the aromatic ring with less uniform charge separation, so phenols are weaker acids.
羧酸是弱酸，其pKa值通常在4–5范围内（例如乙酸pKa = 4.76）。它们的酸性比醇（pKa ≈ 16）和酚（pKa ≈ 10）强得多。较高的酸性归因于羧酸根离子的共振稳定作用：负电荷均匀离域到两个氧原子上，生成两个等价的共振结构。这种离域降低了共轭碱的能量，使酸更易释放质子。相比之下，烷氧根离子缺乏共振稳定，而酚氧根离子虽有共振，但负电荷离域到芳环中，电荷分离不够均匀，因此酚的酸性较弱。
RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺
5. Comparing Acid Strength: Inductive Effects | 酸性强度比较：诱导效应
ic acids is strongly influenced by inductive effects. Electron-withdrawing groups (such as –Cl, –NO₂, – ilise the negative charge of the carboxylate ion by pulling electron density away from the – mes stronger than ethanoic acid. The effect is more pronounced when the electronegative substituent is closer to the carboxyl group, and increases with the number of halogen atoms: CCl₃COOH (trichloroethanoic acid) is a much stronger acid. Conversely, electron-donating alkyl groups (+I effect) slightly destabilise the anion, making longer-chain or branched acids marginally weaker than ethanoic acid.
取代羧酸的酸性强度受诱导效应的强烈影响。烷基链上的吸电子基团（如–Cl, –NO₂, –CN）通过将电子密度拉离–COO⁻基团来稳定羧酸根离子的负电荷，从而增强酸性。例如，氯乙酸（ClCH₂COOH, pKa = 2.86）大约比乙酸强100倍。当电负性取代基离羧基更近时，该效应更为显著，并随卤原子数目增加而增强：CCl₃COOH（三氯乙酸）是强得多的酸。相反，给电子烷基（+I效应）略微使阴离子不稳定，使得较长链或支链酸的酸性略弱于乙酸。
6. Reactions with Metals, Bases and Carbonates | 与金属、碱和碳酸盐的反应
m, they liberate hydrogen gas and form carboxylate salts: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂↑ acid group is the reaction with carbonates (Na₂CO₃) or hydrogencarbonates (NaHCO₃). Effervescence of CO₂ is observed: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑ ction does not occur with alcohols or phenols, making it a reliable functional group test.
羧酸能发生典型的酸反应，但其弱酸性决定了一些差异。与活泼金属如镁反应，它们释放氢气并生成羧酸盐：2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂↑。与氢氧化钠等碱发生简单中和反应：CH₃COOH + NaOH → CH₃COONa + H₂O。羧酸基团的一个关键鉴别实验是与碳酸盐（Na₂CO₃）或碳酸氢盐（NaHCO₃）的反应。可观察到CO₂气泡产生：CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑。醇或酚不发生此反应，因此它是可靠的官能团检验方法。
7. Esterification | 酯化反应
Esterification is the acid-catalysed condensation between a carboxylic acid and an alcohol to form an ester and water. This is a reversible reaction typically carried out with a few drops of concentrated sulfuric acid as a catalyst and heated under reflux. For example, ethanoic acid reacts with ethanol to give ethyl ethanoate: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The mechanism for primary and secondary alcohols follows Fischer esterification, where the acid is protonated, the alcohol attacks the carbonyl carbon, a tetrahedral intermediate is formed, and subsequent proton transfer and elimination of water yield the ester. To drive the equilibrium to the right, either the ester or water is removed, or an excess of one reactant is used. Esters are known for their pleasant fruity smells and are used widely as solvents and flavourings.
酯化是羧酸与醇在酸催化下进行缩合形成酯和水的反应。这是一个可逆反应，通常加入几滴浓硫酸作催化剂，并在回流下加热。例如，乙酸与乙醇反应生成乙酸乙酯：CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O。伯醇和仲醇的反应遵循费歇尔酯化机理：酸被质子化，醇进攻羰基碳，形成四面体中间体，随后经过质子转移和水消除得到酯。为了使平衡向右移动，可以移除酯或水，或者使用过量的一种反应物。酯以其怡人的水果香气著称，广泛用作溶剂和调味剂。
8. Formation of Acyl Chlorides | 酰氯的生成
de (PCl₃), or thionyl chloride (SOCl₂). The reaction with SOCl₂ is often preferred because the by-products (SO₂ and HCl) are gases that escape, leaving a pure acyl chloride: CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl. With PCl₅, the equation is CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl. Acyl chlorides are highly reactive derivatives, readily undergoing nucleophilic addition– of acyl chlorides with water producing a carboxylic acid and HCl is used as a test, but it must be distinguished from simple acid fumes.
羧酸可以通过与五氯化磷（PCl₅）、三氯化磷（PCl₃）或亚硫酰氯（SOCl₂）反应转化为酰氯。常优选与SOCl₂反应，因为副产物（SO₂和HCl）是气体逸出，留下纯净的酰氯：CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl。与PCl₅的反应方程式为CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl。酰氯是高活性衍生物，极易与水、醇、氨和胺发生亲核加成-消除反应，因此它们是有机合成中的关键中间体。酰氯与水剧烈反应生成羧酸和HCl，可用作检验，但需与普通酸雾区分。
9. Reduction to Primary Alcohols | 还原成伯醇
Sodium borohydride (NaBH₄) is not strong enough to reduce the carboxyl group. Four hydrogen atoms are added per acid molecule, requiring LiAlH₄ in a 1:0.25 molar ratio (acid:LiAlH₄), as LiAlH₄ provides hydride ions. For example, propanoic acid is reduced to propan-1-ol: CH₃CH₂COOH + 4[H] → CH₃CH₂CH₂OH + H₂O. The reaction is carried out under anhydrous conditions because LiAlH₄ reacts violently with water. This reduction is important for transforming a carboxylic acid into a primary alcohol, which can then be further functionalised.
羧酸可被强还原剂氢化铝锂（LiAlH₄）在无水乙醚（乙氧基乙烷）中还原为伯醇。硼氢化钠（NaBH₄）的强度不足以还原羧基。每个酸分子加入四个氢原子，因此酸与LiAlH₄的摩尔比约为1:0.25，因为LiAlH₄提供氢负离子。例如，丙酸被还原为丙-1-醇：CH₃CH₂COOH + 4[H] → CH₃CH₂CH₂OH + H₂O。反应须在无水条件下进行，因为LiAlH₄与水剧烈反应。该还原反应对于将羧酸转化为伯醇并进一步功能化非常重要。
10. Decarboxylation | 脱羧反应
he sodium salt of a carboxylic acid with soda lime (a mixture of NaOH and CaO) removes the – ne: CH₃COONa + NaOH (CaO, heat) → CH₄ + Na₂CO₃. This reaction is a classic laboratory preparation of methane gas. For aromatic acids, the conditions are milder; simple aromatic monocarboxylic acids can be decarboxylated by heating with soda lime, but polycarboxylic acids may do so more readily due to additional stabilisation of the incipient carbanion.
脱羧是指从羧酸盐中失去二氧化碳。将羧酸的钠盐与碱石灰（NaOH和CaO的混合物）共热，会移除–COONa基团并用氢原子取代，生成少一个碳的烷烃。例如，乙酸钠脱羧生成甲烷：CH₃COONa + NaOH（CaO，加热）→ CH₄ + Na₂CO₃。该反应是实验室制备甲烷气体的经典方法。芳香族酸的反应条件较温和；简单的芳香族单羧酸与碱石灰加热即可脱羧，但多元羧酸可能因初生态负碳离子有额外稳定而更容易反应。
11. Summary of Key Reactions | 重点反应小结
For quick revision, the following reactions should be memorised:
快速复习时，应熟记以下反应：
Acid + metal → salt + H₂; e.g. 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
酸 + 金属 → 盐 + H₂；例：2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂Acid + base → salt + H₂O; e.g. CH₃COOH + NaOH → CH₃COONa + H₂O
酸 + 碱 → 盐 + H₂O；例：CH₃COOH + NaOH → CH₃COONa + H₂OAcid + carbonate/hydrogencarbonate → salt + CO₂ + H₂O
酸 + 碳酸盐/碳酸氢盐 → 盐 + CO₂ + H₂OEsterification: RCOOH + R'OH ⇌ RCOOR' + H₂O (conc. H₂SO₄, reflux)
酯化：RCOOH + R'OH ⇌ RCOOR' + H₂O（浓H₂SO₄，回流）Acyl chloride formation: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl
酰氯生成：RCOOH + SOCl₂ → RCOCl + SO₂ + HClReduction: RCOOH + 4[H] → RCH₂OH + H₂O (LiAlH₄, dry ether)
还原：RCOOH + 4[H] → RCH₂OH + H₂O（LiAlH₄，无水乙醚）Decarboxylation: RCOONa + NaOH (CaO, heat) → RH + Na₂CO₃
脱羧：RCOONa + NaOH（CaO，加热）→ RH + Na₂CO₃
12. Common Exam Pitfalls | 常见考试陷阱
Students often confuse the acidities of alcohols, phenols and carboxylic acids. Remember the order: carboxylic acid > phenol > alcohol. When writing equations, ensure correct balancing and include state symbols only if asked. In esterification, do not forget the catalyst and that the reaction is reversible. Also, note that NaBH₄ cannot reduce carboxylic acids – only LiAlH₄ is effective. When comparing acid strengths, always mention the stability of the conjugate base and refer to delocalisation or inductive effects rather than simply stating pKa values. In reactions with carbonates, be aware that phenols do not react, so this is a good distinction.
学生常常混淆醇、酚和羧酸的酸性顺序。请记住：羧酸 > 酚 > 醇。书写方程式时，确保正确配平，仅在有要求时才加状态符号。在酯化反应中，不要忘记催化剂，并且该反应是可逆的。还要注意，NaBH₄不能还原羧酸——只有LiAlH₄有效。比较酸性强度时，务必提及共轭碱的稳定性，并引用离域或诱导效应，而不是仅仅陈述pKa值。在与碳酸盐的反应中，要知道酚不反应，这是很好的区分方式。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图